Dual‐Gas Stability of RAB‐Joints in SOFC Applications

Abstract

The gas-tightness of metal/ceramic seals is a key requirement for reliable long-term operation of SOFC stacks. For the application in light-weight automotive devices like auxiliary power units (APU) in particular the resistance of the joints to rapid thermal cycling is of essential importance to meet customer expectations and to ensure longevity of the APU sub-system. [1] Post-mortem analysis of operated full scale SOFC-stacks and stack components is an essential part of the development process, reaching from material selection, [2,3] component design and construction, to stack assembly and operation. For deeper understanding of full-scale stack posttest analysis results it is often useful to develop a basic damage mechanism hypothesis applying model systems of reduced complexity. For this reason, thermomechanical testing of simplified specimens in realistic thermochemical environment is considered to be a key contribution to boost feedback from post-test analysis into the development cycle. Light Weight Stack Design for Automotive ApplicationSolid oxide fuel cells consist of multiple layers (anode, electrolyte and cathode). In case of applying a planar design the cells are sealed into metallic frames ("upper manifold sheet" in Figure 1) to form so called "repeating units" (Figure 1). Several of these are piled up and electrically connected in series to form a stack of practical voltage. For example stacks with planar solid oxide fuel cells are being developed for the application in APUs for vehicles. [1] 2. Motivation -From Full-Scale to Model System Experiment and BackAs a first step, full-scale stacks were examined for microstructural changes of the seals (produced by reactive air brazing (RAB) utilizing a Ag8CuO braze for the cell/upper manifold sheet joints and Ag4CuO brazes for the upper manifold sheet/support frame joints, cf. Figure 2) caused by thermochemical/thermomechanical wear after real-life stack operation. In some areas of stacks the seals displayed pronounced pore formation within the silver matrix of the braze.As a second step, specific changes (isothermal, thermal cycling, changes in superimposed external mechanical load) were introduced into the operation conditions of several fullscale trial stacks to study the effect on pore formation in braze joints. Due to interaction of a multitude of parameters these experiments did not yield definite results in every case.For this reason, a third step towards understanding the phenomenon of pore formation was undertaken: On the basis of model experiments, utilizing simplified material systems and testing configurations, various real-life damage patterns were provoked by intentional changes in experimental conditions. Finally, these damage patterns were correlated with the ones encountered in the full-scale stacks to enable targeted assessment of the proposed damage mechanisms. Experimental Reactive Air BrazingRAB became increasingly important in recent years and is an alternative to vacuum brazing. The RAB-process was developed from "Direct copper bondi...

Aging Behavior of Reactive Air Brazed Seals for SOFC

“…Figure 8 shows an overview on the microstructure of a Crofer/3YSZ joint brazed with Ag‐4CuO after aging in dual atmospheres. The loss of the gas‐tightness might be attributed to the formation of pores in the brazing seam and to the destruction of the 3YSZ, which is worse than the results published in 25 for aging at 750 °C. However, in contrast to the results published in 20 and 25, during aging in dual atmosphere at 850 °C the pore formation mechanisms plays a minor roll compared to the accelerated aging caused by the higher temperature.…”

Forschungszentrum Jülich – Progress in SOC Development

“…Based on experience with the F-design, thermo-mechanical robustness has been improved, which is particularly important since, for example, for APU applications in vehicles, heating times to operating temperature of 30 minutes and less are required. A known weak point of the cassette design is the insufficient durability of the silver solder hitherto used for cell soldering, as noted by Brandenberg et al (9). By adapting the design, the cells could be joined to the frame with glass-ceramic, which promises much better long-term stability than the silver solder.…”

Characterization of Ag-Based Filler for Electrolyte/Metallic-Interconnect Joint in Metal-Supported Solid Oxide Fuel Cells Applications

“…For decades, related brazing studies mainly focused on the joining between YSZ electrolyte and ferrite-based metal-interconnect, evolution of interfacial microstructures, and hermetic properties after exposure to long-term operation at elevated temperatures (17)(18)(19). Recently more researches have been concentrated on tailoring the thermal expansion of Ag-based fillers by mixing ceramic particles (20)(21).…”